1. Field of the Invention
This invention relates to an apparatus and method for increasing the reactivity of a fuel/air mixture prior to homogenous combustion of the mixture. More specifically, the present invention is a pilot for a gas turbine combustor which utilizes the heat of combustion within the pilot to increase the reactivity of a portion of the fuel/air mixture utilized by the pilot.
2. Brief Description of the Related Art
Known dry low NOx combustion systems for gas turbines can achieve relatively low emissions levels; however, the use of continuous pilot systems, as distinguished from starter systems, is required to stabilize combustion over a wide range of gas turbine operational conditions and minimize emission levels.
U.S. Pat. No. 5,634,784 represents a state-of-the-art continuous pilot. The patent teaches a catalytic pilot that will make a portion of the fuel/air mixture destined for the pilot's combustion zone more reactive by passing it through a catalytic centerbody. The patent also teaches that by recirculating hot combustion gas products back on to the catalytic centerbody the catalytic centerbody can use the heat of combustion within the pilot to assure that the catalytic component of the centerbody is at a suitably high operating temperature.
The structure of the catalytic centerbody design previously taught has several shortcomings. In particular, no method is provided to limit the temperature of the centerbody, thus the surface temperature could reach the adiabatic flame temperature of the fuel/air mixture, generally above the centerbody's material failure temperature. In addition, the short channel design of the catalyst limits the residence time thus the catalytic reaction of the centerbody fuel/air mixture, which is a critical factor in assuring flame stability within the pilot.
It has now been found that by utilizing the fuel/air mixture passing through the centerbody more fully, a more versatile pilot can be created. The invention accomplishes this by increasing the channel length for the fuel/air mixture within the centerbody. In addition, the fuel/air mixture entering the centerbody is given the function of cooling the centerbody structure, increasing the temperature and overall combustibility of the fuel/air mixture, and allowing the centerbody to be exposed to greater temperatures, even temperatures above the material limit of the centerbody. The improved centerbody can be used to either impart a thermal rise to the fuel/air mixture, a degree of catalytic pre-reaction, or a combination of both.